Aspirin (acetyl salicylic acid) effectively reduces the risk of secondary thrombotic events in individuals who have experienced angina, myocardial infarction, peripheral artery disease, or cerebrovascular ischemia. Aspirin also may reduce the risk of initial thrombotic events in healthy individuals. For this reason, many individuals, through physician prescriptions or self-medication, take aspirin on a regular basis for the primary or secondary prevention of thrombotic disease.
Some important questions remain unanswered. What long-term dosage of aspirin is likely to confer protection from thrombosis while avoiding gastric discomfort or hemorrhagic conditions? What individuals are “resistant” to aspirin, how may aspirin resistance be identified, and what effect does aspirin resistance have on the interpretation of clinical trials?
Several studies suggest that aspirin, at doses between 30 and 325 mg/day, is effective in reducing the incidence of arterial thrombotic events. Physicians customarily prescribe aspirin to prevent myocardial infarction, cerebrovascular thrombotic disease, and vascular death in individuals with stable angina (1), unstable angina (2), myocardial infarction (3), transient cerebral ischemia, peripheral vascular disease, and thrombotic stroke. (4) For patients with prior acute myocardial infarction or stroke, aspirin prevents up to 40 new thrombotic events per thousand treated. (5) Two large randomized trials demonstrate the thrombosis-reducing efficacy of aspirin for patients with acute ischemic stroke, and aspirin is now routinely prescribed in these conditions. (6,7)
Aspirin has been shown to be effective in prevention of heart disease in men with several high risk clinical conditions and both men and women with hypertension. (8,9) The Program on the Surgical Control of the Hyperlipidemias categorized a number of individuals who had a partial ileal bypass surgery as smokers and non-smokers and assigned them to aspirin and non-aspirin arms. Within this population, the overall mortality rate was 45.2% for smokers with no aspirin use and only 10.4% for those who reported even infrequent aspirin use. (10)
Primary Prophylaxis with Aspirin
Aspirin reduces the incidence of thromboembolic arterial disease in healthy individuals over 50 years of age. (11, 12, 13) In healthy men, aspirin appears to prevent an average of four thrombotic events per thousand subjects treated. In the Physicians' Health Study, healthy men who took 325 mg of aspirin every other day experienced a mean reduction in the incidence of first myocardial infarction of 44.8% compared to those taking placebos. (14) There was a particularly marked reduction of 59.3% for the morning peak of infarction, between 4 and 10 a.m. These findings were not confirmed in the British Doctors' study, however, and additional confirmatory studies are in progress, including the current Women's Health Study. (15)
Aspirin Dosage and Complications
Hemorrhage and Gastrointestinal Toxicity
There is a dose-related risk of gastrointestinal bleeding in aspirin therapy, especially in patients who have coagulopathy or who are taking additional anticoagulant therapy such as heparin or warfarin. (16) This is of particular concern in individuals with stomach lesions such as the ulcers associated with Helicobacter pylori infection. To avoid hemorrhage, many physicians recommend enteric-coated aspirin, especially if the recommended dosage exceeds 325 mg/day. (17) In the British Doctors' Study, where the prescribed randomized dose was 500 mg/day, 20% of aspirin arm participants dropped out due to dyspepsia or constipation, 3.6% experienced bleeding or bruising, and 2.2% had gastrointestinal blood loss. (18)
In the Cardiovascular Health Study, Kronmal, et al found a 1.6× relative risk of ischemic stroke and a 4× relative risk for hemorrhagic stroke for healthy women 65 and older who took aspirin. (19) This study presents self-reported, not randomly assigned, aspirin users and illustrates the hemorrhagic risk of high-dose aspirin in particular disease situations.
For patients who have an elevated risk of thrombosis, the absolute benefit of aspirin prophylaxis clearly outweighs the relatively small risk of bleeding. However, the individuals with no risk factors, aspirin dosages must be carefully monitored to avoid gastric discomfort, gastric hemorrhage, systemic hemorrhage and hemorrhagic stroke.
Recommendations for Therapeutic Aspirin Usage
Because of the preponderance of evidence favoring the protective effects of aspirin against arterial thrombosis, millions rely on aspirin prophylaxis daily either by physician's prescription or self-medication. In a review of clinical studies undertaken in the 1980s and 1990s, Hirsh, Dalen, Fuster, et al make the following recommendations: (4)                Aspirin is indicated for patients with stable angina, unstable angina, acute myocaridal infarction, transient cerebral ischemia, thrombotic stroke, and peripheral arterial disease.        A dose of 75 to 100 mg/day should be used chronically for all indications, although an initial dose of 160 to 325 mg should be used in acute settings.        For patients with cerebrovascular disease, a dose of 75 mg/day is effective.        Aspirin at 100 mg/day is indicated for patients with prosthetic heart valves who develop systemic embolism while on warfarin.        Aspirin is indicated for patients with atrial fibrillation in whom warfarin is contraindicated.There is no recommendation either for or against aspirin usage in normal, healthy adults, and no standards have been established for laboratory monitoring of aspirin's efficacy, a growing concern as clinicians became aware of aspirin resistance and interindividual pharmacokinetic variations.        
Recent clinical investigations indicate that approximately 10% to 15% of patients on aspirin therapy for prevention of thrombosis have a less than adequate platelet suppression response. (20) Additionally, it is reported that some individuals develop an increasing resistance over time. (21) It is uncertain if this observed resistance is the result of poor absorption, changes in pharmacodynamics, non-compliance, or mechanisms not currently identified. Ridker, et. al. reported that the positive effect of aspirin appears to be altered by underlying inflammation and further suggested that markers of inflammation such as C-reactive protein may delineate those individuals who will respond atypically to aspirin. (22)
Platelet Activity Studies and Variable Aspirin Response
Current laboratory measures of platelet activation include the Mielke template bleeding time, aggregometry, lumiaggregometry, the Dade-Behring PFA-100 platelet function analyzer, and the platelet reactivity test. (23) While the platelet-suppressing property of aspirin clearly affects the results of these procedures in general, individuals' laboratory responses to aspirin therapy are idiosyncratic. Trip et. al. report 46% of patients with positive spontaneous platelet aggregation results suffered a repeat myocardial infarction. (24) This was the first study that paired clinical outcomes with laboratory results. Helgason, et. al. demonstrated that seven of 17 patients with atrial fibrillation achieved only partial inhibition of platelet aggregation when taking 325 mg of enteric coated aspirin per day. Non-compliance had no statistical effect on this study's outcome. Helgason's group further demonstrated that 8.2% of patients with previous ischemic stroke exhibited restoration of platelet aggregation (aspirin resistance) despite escalation of aspirin dosage to, ultimately, 1300 mg/day. (25)
Pappas, et al used a specially designed visual platelet aggregate inspection technique to measure the response to the inhibitory effects of aspirin in 31 healthy young adults. (26) They demonstrated a wide inter-individual variation in response to aspirin that was consistent over 28 days of aspirin ingestion. Mueller, et al performed corrected whole blood aggregometry on patients with intermittent claudication who were taking 100 mg aspirin/day after elective peripheral balloon angioplasty. (27) At any given time, only 40% of males showed complete inhibition of aggregation. Significantly, non-responsive aggregometry results in this study predicted increased risk of reocclusion, leading to the conclusion that aspirin may fail to protect partial-and non-responders from occlusive events.
Grotemeyer, et al, performed whole platelet reactivity tests on 180 internal carotid artery stroke victims given 500 mg of aspirin three times per day. (28) All began with elevated platelet reactivity immediately following stroke. Upon initial aspirin administration, 90% of the subjects demonstrated an immediate suppression of platelet reactivity. However, 60 patients' platelets resumed enhanced platelet reactivity only twelve hours after the initial aspirin dosage. These were termed secondary aspirin non-responders. Over a 24-month period following discharge, 24 (40%) of the secondary aspirin non-responders experienced myocardial infarction, repeat stroke, or vascular death (p<0.0001). Of 114 remaining subjects (six were lost to follow-up), only five (4.4%) suffered these major endpoints. Grotemeyer concluded that early identification of secondary aspirin non-responders is an important step to effective prevention of further thrombotic events in post-stroke patients. In an extensive review of aspirin and platelet laboratory studies, Patrono, et al, commented that 10% to 15% of individuals have a poor initial response or demonstrate progressive resistance to aspirin. No large clinical trials have incorporated laboratory measures of platelet activation, so the effect of aspirin resistance on clinical outcomes is currently unknown.
Komiya, et al used platelet aggregometry to detect cases of aspirin therapy non-compliance and incorrect dosage. They found that 10% of 159 outpatients' results were outside the diagnostic parameters because of non-compliance and that an additional 2% were confirmed compliant but still had normal aggregometry results. (29) Their study illustrates the necessity for monitoring aspirin therapy in patients who may be suspected of non-compliance.
Aspirin Suppresses Platelet Activity
Agonists Trigger Platelet Activation
The cyclooxygenase (COX) biochemical activation pathway, as diagrammed in FIG. 1, is essential to normal platelet activation and to the prevention of systemic hemorrhage. (30) COX is also an important activation enzyme in other cells.
Activation begins when a platelet agonist such as ADP, epinephrine, collagen, or thrombin binds to its platelet membrane receptor site. This activates phospholipase A2, a membrane-associated enzyme, and frees arachidonic acid, a 20-carbon unsaturated fatty acid, from its supporting membranes phospholipid. Free arachidonic acid is a substrate for the COX pathway. (31)
The Platelet Cyclooxygenase Pathway
COX, a membrane-associated endoperoxide synthase with two catalytic sites, rapidly modifies the free arachidonic acid in a two-step process. (32) The first catalytic site converts it to the endoperoxide PGG2. The second site, a peroxidase-type site, converts the short-lived PGG2 to PGH2. PGH2 is then converted by the isomerase action of thromboxane synthase to thromboxane A2 (TXA2), which activates the platelet.
TXA2 is rapidly hydrolyzed to thromboxane B2 (TXB2), a stable plasma product of the COX pathway. TXB2, in turn, is converted to a variety of end products, most of which are excreted via the kidney. (33)
Aspirin Irreversibly Acetylates Cyclooxygenase
Platelets (and other cells) are now known to produce two isoforms of COX-1 and COX-2. (34) COX-1 is a constitutive membrane-bound enzyme that functions in all normal platelets, whereas COX-2 is a cytokine-inducible enzyme that appears in newly produced platelets and in other cells during inflammation.
Aspirin irreversibly acetylates both COX-1 and COX-2 at serine 529, see FIG. 2. For the COX-1 enzyme, the attached acetyl group sterically hinders arachidonic acid's access to its reactive site. Acetylation does not appear to hinder the activity of COX-2. The inflammation-induced activity of COX-2 in platelets may account for some cases of aspirin resistance (35), as may pharmacokinetic variations among individuals.
Aspirin Pharmacokinetics
Aspirin is rapidly absorbed from the stomach and duodenum and is rapidly hydroyzed to salicylic acid by esterases in the gut, liver, and erythrocytes. Because only aspirin, not salicylic acid, acetylates COX-1 (or COX-2), a significant proportion of acetylation occurs in the presystemic circulation of the gut and liver. (36) Salicylic acid circulates bound to plasma proteins for up to six hours and is cleared by the kidney.
Platelets acetylated during the time of peak aspirin levels lose most of their ability to be activated; as reflected in prolonged bleeding times, reduced aggregometry responses, and diminished TXB2 production. A single dose of 325 mg aspirin is detectable within minutes using these laboratory assays and the effects remain for six to ten days. Platelets with acetylated COX-1 survive normally and continue to participate in the adhesion reaction. Normal platelet function test results are restored only when a predominant population of new platelets has been released from the bone marrow.
Additional Platelet Activity-Suppressing Substances
Other non-steroidal anti-inflammatory drugs (NSAIDs) such as dipyridamole, sulphinpyrazone, and ibuprofen act upon COX-1 or other platelet enzymes, but no clinical trials have established antithrombotic properties for these drugs. Ticlopidine, clopidegril, and abiximab exert their antiplatelet activity on membrane receptors. The effects of all these therapeutics may be measured using aggregometry, lumiaggregometry, and plasma, serum, or urine TXB2 assays.
In addition, many dietary components and supplements have been shown to modify platelet function by unknown mechanisms. These include fish oil (37), vitamin E (38), garlic (39), red wine, and purple grape juice (40). Herbal and dietary supplements may both interfere with or enhance the effect of aspirin on platelets.
Janssen, et al provided healthy volunteers with 3 mg of aspirin per day, a study designed to mimic acetylsalicylic acid levels in certain plants. (41) They showed that serum TXB2 levels were reduced by 39% compared to placebo.
A number of prospective randomized clinical trials have demonstrated that 50 to 500 milligrams of aspirin per day effectively reduces the risk of primary or secondary arterial thrombotic events in many individuals. Recognition of aspirin's antithrombotic properties has prompted clinical researchers to focus substantial efforts towards modifying platelet function with not only aspirin but a variety of newly developed platelet-suppressive drugs.
Despite the availability of these new drugs, the use of aspirin for arterial thrombosis prevention continues to increase as public awareness grows. More than 80 billion aspirin tablets are consumed annually in the USA, and more than 37% of the individuals taking aspirin do so to “prevent blood clots”. Additionally, individuals who employ alternative medicine practices may consume significant quantities of red wine, purple grape juice, fish oil, vitamin E, garlic, ginkgo biloba, and other substances known to interfere with platelet function.
Numerous reports in the scientific literature detail varied individual responses to aspirin dosages. However, little is reported regarding the potential need to adjust aspirin dosage according to individual biologic response or to a changing response over time. Limited studies correlate the occurrence of thrombotic events with individuals who become “resistant” to aspirin.
Subject381214171824% Inhibition (81)60784648701710% Inhibition (325)44408283468338
Why hasn't aspirin, the drug most widely consumed to modify platelet function, been dosed according to biologic response, in a manner similar to anticoagulants? Because most laboratory tests used to monitor platelet function are time consuming, require expensive equipment (platelet aggregometer), or are traumatic to the patient (bleeding time). In addition, all current assay methods are subject to wide variation in results due to preanalytical and analytical variables, inherent in all ex vivo tests that require the patient's platelets.